Interferometers known as multiple beam interferometers are low transmissivity devices which make use of what is called multiple beam interference, which is described in detail in Max Born and Emil Wolf, Principles of Optics, Sixth Edition, 1980, Pergamon Press, Inc., Elmsford, N.Y. 10563, pp. 323-33, 350-58, incorporated by reference herein. These interferometers are used, for example, as wave-meters to measure the wavelength of incident radiation; as filters to transmit only desired portions of the incident radiation; and as discriminators to respond to changes in the incident radiation. See Benedikt Faust and Lennart Klynning, "Low-cost wavemeter with a solid Fizeau interferometer and fiber-optic input," Applied Optics, Vol. 30, No. 36, 20 Dec. 1991, pp. 5254-59; W. Moos, G. F. Imbusch, L. F. Mollenauer and A. L. Schalow, "Tilted-Plate Interferometry with Large Plate Separations," Applied Optics, Vol. 2, No. 8, August 1963, pp. 817-22; D. F. Gray, K. A. Smith and F. B. Dunning, "Simple compact Fizeau wavemeter," Applied Optics, Vol. 25, No. 8, 15 Apr. 1986, pp. 1339-43; Mark B. Morris, Thomas J. McIlrath and James J. Snyder, "Fizeau wavemeter for pulsed laser wavelength measurement," Applied Optics, Vol. 23, No. 21, 1 Nov. 1984, pp. 3862-68; Leo J. Cotnoir, "Stand-alone instruments measure laser wavelengths," Laser Focus World, April 1989, pp. 109-20; Christopher Reiser, Peter Esherick and Robert B. Lopert, "Laser-linewidth measurement with a Fizeau wavemeter," Optics Letters, Vol. 13, No. 11, November 1988, pp. 981-83; H. D. Polster, "II. Multiple Beam Interferometry," Applied Optics, Vol. 8, No. 3, March 1989, pp. 522-25, all of which are incorporated by reference herein. In addition, these interferometers have been used as spectrum analyzers, wavelength-division multiplexers, and in other related applications.
The measurement of the wavelength of optical radiation with a precision of better than 1 part in 10.sup.6 generally requires the use of interferometric instrumentation. For this purpose, there are two generic types of interferometric instruments, scanning and fixed. Scanning instruments are not considered here, because they are only well suited for use with continuous wave (CW) sources. Fixed interferometers can be used with either pulsed or CW light sources, and are useful for a wide variety of applications.
A multiple beam interferometer is termed an etalon when the spacing between its plates or mirrors is "fixed" or not adjustable. Multi-beam etalons are generally categorized as either Fabry-Perot etalons, which include mirrors that are parallel-spaced, as in FIG. 1A, or Fizeau etalons, which include mirrors that are wedged-spaced as in FIG. 2A.
FIG. 1A illustrates a multibeam interferometer 1 including a parallel etalon 2 which is illuminated with divergent light 3a originating from a source 3b. The etalon 2 forms circularly symmetric fringes at infinity by multiple beam interference. These fringes possess intensity maxima at angles .theta. such that t cos .theta. is an integer number of half-wavelengths .lambda./2, where .theta. is the angle of incidence at the etalon 2 with respect to the optical axis and t is the optical path length between mirrors 2a and 2b of the etalon 3. Focusing optics 4 image the fringes received from the etalon 2 onto a focal plane 5 as fringes 5a, as shown in FIG. 1B, at integer numbers of half wavelengths .lambda./2, where K and K+1 represent two successive integers. Light which is not transmitted through the parallel etalon 2 of the interferometer 1 diverges upon reflection and is not available for further use.
FIG. 2A illustrates an interferometer 10 including a wedged-spaced etalon 11 which is illuminated with collimated or parallel light supplied from a collimating optic or lens 12 which receives light from a source 13. Straight fringes 14 are formed by the etalon 11 and have intensity maxima occurring where the effective optical path between mirrors 11a and 11b of the etalon 11 is an integer number of half-wavelengths. FIG. 2B illustrates two such fringes 14a and 14b, corresponding to successive integers K and K+1, as they may be observed in transmission at any plane beyond the etalon 11, without the need for additional optics. It is well known that optics may be used to modify the fringe spacing or to collapse the straight fringes into high-intensity spots.
In conventional applications of the configurations for the interferometers illustrated in FIGS. 1A and 2A, the light that is not transmitted through the interferometer is reflected back but not otherwise utilized. For the parallel-mirror etalon 2 of the interferometer 1, the incident light is divergent, so that the reflected light continues to diverge and cannot be further employed. However, the wedged etalon 11 of the interferometer 10 is illuminated with collimated light, so that the reflected light remains collimated but nevertheless also unutilized.